24 PLANORBIS DILATATUS. 



DESCRIPTION. 



Shell small, delicate, wide, carinated, with 

 the hues of increment fine; closely umbilicated 

 below; slightly convex above, with the suture 

 conspicuous, and the apex slightly impressed: 

 body whirl enlarging rapidly, and losing the 

 well defined carina as it approaches the peri- 

 treme, which, however, is still somewhat modi- 

 fied by it. 



Color light brown, translucent. 



Geographical Distribution. Massachusetts, 

 Ohio, ? Delaware river, Maryland. 



OBSERVATIONS. 



More closely rolled, and wider transversely 

 than P. exacutus or P. parvus; and distinguished 

 from both by the narrow umbilication of the 

 left side. The right, or upper side, resembles 

 that of P. exacutus, but the margin is more ob- 

 tusely carinated, and the edge of the peritreme 

 is not so convex. 



Herr Pfeiffer has described an European 

 Planorbis dilatatus in Wiegmann's Archiv. fiir 

 Naturgeschichte, 1841, p. 225. If it be a true 

 species, it will be rather difficult to determine 



